The present invention relates to the art of electrical activation of the diaphragm using epimysial electrodes.
It is to be appreciated that the invention is also applicable to control other nerve groups through electrical activation using epimysial electrodes.
It is possible to support ventilation in patients with chronic ventilatory insufficiency using a diaphragm pacer device. In broad terms, these devices pass a small amount of electric current through a pair of electrodes placed on the diaphragm muscle itself. The current passes through the diaphragm muscle thereby activating the phrenic nerves which are proximate the placement of the electrode pair, on the opposite side of the muscle. A proportion of the current may also pass through tissues other than the phrenic nerve and thus have no effect on the phrenic nerves. Since the proportion of current affecting phrenic nerve activation depends on the distance between the electrode and the phrenic nerves, electrode placement is critical. In any case, by passing the small amount of electric current through the pair of properly placed electrodes on the diaphragm muscle, the phrenic nerves are activated in turn causing a contraction of the diaphragm muscle, the primary muscle used for breathing. The diaphragm muscle contraction draws air into the lungs and the patient is ventilated. Although the above approach appears to be simple, past attempts at diaphragm pacing have met with limited success for a variety of reasons.
The general idea of stimulating the phrenic nerve for ventilation was investigated by Sarnoff in the late 1940's (Sarnoff 1948). Sarnoff's work was made a clinical reality by Glenn in the 1960's (Glenn 1973, Glenn 1988). In those systems, the electrodes are "cuffs" made of silicon rubber with platinum stimulating surfaces which are placed directly on the nerve in the neck or thorax. The electrodes themselves may be unipolar or bipolar but the current trend is to implant electrodes of the unipolar type. FIG. 1A illustrates a unipolar electrode 10 for diaphragm pacing. FIG. 1B illustrates a bipolar electrode 20 also for diaphragm pacing. The unipolar electrode 10 includes a first electrode 12 for placement around the phrenic nerve and an anode 14 located somewhere in the body of the patient. A pulse of current is applied to a first connector 16, through the first electrode 12. The current passes through the body tissues into the anode 14 and out of the diaphragm pacing system 10 through a second connector 18. A reversal of the above-described flow occurs for a second current pulse immediately following the first. After a short delay, the pair of current pulses is repeated. This repetition continues as long as diaphragm muscle contraction is desired. Current flowing from the first electrode 12 to the anode 14 or from the anode 14 and to the first electrode 12 stimulates the phrenic nerves inbetween.
FIG. 1B illustrates a bipolar electrode wherein a first electrode 22 is electrically isolated from a second electrode 24 using suitable insulating material. As illustrated in the FIGURE, the phrenic nerve is held between the first and second electrodes 22, 24 by placing a suture in the electrode extensions 23, 25. The first electrode 22 is connected to a first connector 26 and the second electrode 24 is connected to a second connector 28. As with the system illustrated in FIG. 1A, current may be applied between the electrode pair 22, 24 in either polarity for suitable activation of the phrenic nerves.
Although the above-described systems perform adequately, a number of problems exist. One problem is that the implant procedure for installing either the unipolar electrode 10 or the bipolar electrode 20 is difficult and invasive. Also, the electrodes themselves impose a risk of irreversible damage to the phrenic nerve through contact therewith. As a result, the devices of FIGS. 1A and 1B have not been well accepted in the medical community. Unfortunately, as a result, the full potential of diaphragm pacing has not been realized through the reluctance to accept these devices.
Another diaphragm pacer is illustrated in FIG. 1C. This pacer 30 uses a similar type of electrode as the device of FIG. 1A placed in a similar location at the phrenic nerve. However, the pacer 30 illustrated in FIG. 1C uses four electrodes 32 placed around the phrenic nerve 34 in a manner slightly different than that possible with the unipolar electrode 10 or the bipolar electrode 20. The main difference in the system illustrated in FIG. 1C is that a four pole sequential nerve stimulation is possible through selective stimulation of pairs of electrodes 32. Also, the pacer 30 is capable of changing anode and cathode configurations of the electrodes during stimulation. Although the pacer 30 offers significant advantages over the earlier described systems, the basic problems remain including the difficulty in implanting the apparatus, the invasive nature of the surgery and the possible risk of irreversible damage to the nerve.
Accordingly, other systems have been developed for electrical activation of respiratory muscles by methods other than phrenic nerve cuff electrodes. Some of these systems are described in "Electrical Activation of Respiratory Muscles by Methods Other Than Phrenic Nerve Cuff Electrodes", D. K. Peterson, T. Stellato, M. L. Nochomovitz, A. F. DiMarco, T. Abelson and J. T. Mortimer, Diaphragm Stimulation Symposium at Cardiostim 1988, Jun. 15-18, 1988, pages 854-860.
With reference now to FIG. 1D, a prior art intramuscular diaphragm stimulating electrode is illustrated. The electrode is shown, extending from the tip of a hypodermic needle 42. The electrode itself is comprised of a polymer barb set 44, a monofilament barb 46 and a coiled multistrand stainless steel wire with teflon insulation. The insulation of the wire ends at the barbs allowing electrical contact between the bare wires and the surrounding tissue. The polymer barb set 44 and the monofilament barb 46 are connected to an associated electrical stimulating device by an extension of the monofilament that passes through the center of the coil of wire 48. The wire 48 carries current from the electrical apparatus to the polymer barb set and the monofilament barb.
In use, the intramuscular diaphragm stimulating electrode 40 is urged down through the hollow body of a hypodermic needle far enough to permit the monofilament barb 46 to spring radially away from the wire 48. The wire is then retracted leaving only the monofilament barb exposed at the tip of the hypodermic needle. The monofilament barb indicates the depth of the needle insertion into the diaphragm during device implant. Although this system provides excellent results, placement of the electrode itself is critical to the success of the procedure. To help with placing the electrode, a laparoscope is inserted into the abdominal space and the surface of the diaphragm itself is observed to locate an appropriate implant site. Optimal theoretical placement of the electrode is known using a "map" of the diaphragm based on anatomical landmarks. However, it is often impossible to tell between patients which point would be most optimal because anatomical landmarks are patient dependent. However, equipped with the barbed intramuscular diaphragm stimulating electrode 40 illustrated in FIG. 1D, surgeons do not have the luxury of multiple attempts at locating an optimal site.
The present invention contemplates a new and improved technique for phrenic nerve stimulation and location of implant sites for diaphragm pacing electrodes.